1. Field of the Invention
The present invention relates to a golf club shaft, and more specifically, to a structure of a golf club shaft which is lighter in weight and which allows a greater degree of freedom in areas of kick point design, bend point, and weight distribution.
2. Description of the Background Art
In order to realize lighter weight for a golf club shaft, either a material having a lighter specific gravity must be employed for the shaft or the shaft must be formed with less material when using a material having the same specific gravity as that conventionally used.
The material for the golf club shaft is selected based on such considerations as strength, modulus of elasticity, cost, possibility for mass production, etc. The known examples of shafts for golf clubs conventionally commercially available include a steel shaft utilizing an iron-based material such as carbon steel and steel alloy, a metal shaft utilizing a metal other than iron-based metals, such as titanium and titanium alloy, and a fiber reinforced plastic shaft (hereinafter referred to as an FRP shaft) using mainly epoxy resin as a matrix and reinforced by fibers such as carbon fibers and glass fibers serving as a reinforcing material.
With a metal shaft, there is a limit to achieving lighter weight since the specific weight of the material used for a metal shaft is generally heavier than that of the material used for an FRP shaft. As a consequence, the recent trends show an increase in the percentage of FRP shafts used: for instance, almost all of the wood clubs has come to employ FRP shafts, and no less than 80% of iron clubs has come to utilize FRP shafts.
The performance of a golf club shaft is evaluated with respect to kick point, bend point, mechanical strength, etc. Here, the term xe2x80x9ckick pointxe2x80x9d refers to the position at which the shaft bends most flexibly. The term xe2x80x9cbend pointxe2x80x9d signifies the characteristic categorized by the bending state of the shaft at a swing, such as being bent flexibly relatively easily in the region in the vicinity of the tip portion of the shaft, as being bent flexibly relatively easily in the region of the shaft near the grip of the club, and as being of the intermediate bending state of the former two cases.
In most cases, the metal shaft is formed of a single material so that the material has a uniform modulus of elasticity, causing the performance of the shaft in areas of kick point, bend point, strength, etc. to be substantially determined by the outer diameter distribution and the thickness distribution in the lengthwise direction of the shaft. On the other hand, the FRP shaft encompasses countless possible choices of strengths of reinforcement fibers, degrees of elasticity, etc., and by suitably combining these choices, the performance of the shaft such as its bend point, strength, etc. can be changed while the outer diameter distribution and the thickness distribution in the lengthwise direction of the shaft remain the same.
The weight of the entire shaft, however, can only be lightened by reducing the amount of material used. Under the circumstance, the improvement of shaft performance such as its strength and bend point involves great difficulties.
Particularly, for the lightweight shaft having an overall weight of about 30 to 40 grams, ensuring the necessary strength is the best that can be done, leaving little room for the consideration of shaft performance such as bend point.
Consequently, there has been a need for a design or a manufacturing method which ensures the necessary strength while achieving lighter weight and which further allows some degrees of freedom in areas of shaft performance, such as bend point.
As described above, a metal shaft has already reached its limits with regard to achieving light weight due to the uniformity of its material. Moreover, with an FRP shaft, it has been the case, when the shaft is a lightweight shaft of about 30 grams to 40 grams as described above, that ensuring at least the required strength for the shaft is the best that can be done, and not enough degree of freedom of design is allowed to realize a special bend point or kick point.
One approach in designing the bend point, the kick point, and the like concerns xe2x80x9cbending stiffnessxe2x80x9d (EI). Here, E is the Young""s modulus (modulus of elasticity), which is dependent upon the solid state properties of the material forming the shaft. I is the geometrical moment of inertia, which is proportionate to the biquadrate of the outer diameter of the shaft. Bending stiffness is a product of these two elements, E and I.
Based on the approach described above, a technique of achieving lighter weight while changing the bend point and the kick point by employing a reinforcing fiber of high elasticity for the FRP shaft to increase the value of E is contemplated. A high elasticity fiber, however, is quite expensive, and, despite its high cost, only little effect can be expected of the high elasticity fiber to change EI. This is due to the fact that, while I is proportionate to the biquadrate of the outer diameter, an increase in E is only reflected as an increase in the modulus of elasticity proportionate to the value of E.
Since I is proportionate to the biquadrate of the outer diameter, it is not altogether impossible to change the performance in the areas of bend point, kick point, etc. with extreme outer diameter distribution and the thickness distribution. In order to maintain the strength of the shaft, however, portions designed with extreme outer diameter distribution and the thickness distribution must be reinforced, resulting in the increase in the overall weight of the shaft.
Under such circumstance as described above, there was no solution but to wait for the development of a new inexpensive material or fiber of high strength or to develop a novel measure to improve the structure and the manufacturing method in order to achieve lighter weight as well as satisfactory performance in the areas of bend point, kick point, and the like.
In view of the above problem of prior art, the main object of the present invention is to provide a golf club shaft which is designed to be lighter in weight without reducing the mechanical strength yet reducing the material cost, while allowing a greater degree of freedom in designing the characteristics such as bending stiffness distribution, a kick point position, bend point, etc.
The golf club shaft according to the present invention that achieves the above objective is provided with a PCCP (Pseudo-Cylindrical Concave Polyhedral shell) structure on its outer circumferential surface. According to the present invention, the PCCP structure may be either provided on the entire outer circumferential surface of a golf club shaft, or provided only partially on the outer circumferential surface. When the PCCP structure is provided on the entire outer circumferential surface, it may be provided not only in one location but in multiple locations.
In one embodiment of the golf club shaft according to the present invention, the PCCP structure is provided at least in a portion within the range of 150 mm to 400 mm from a grip end of the shaft and/or at least in a portion within the range of 0 mm to 350 mm from a neck upper end of the shaft. Such provision of the PCCP structure can form a kick point in the vicinity of the grip end of the shaft and/or a kick point in the vicinity of the tip portion of the shaft.
A kick point can also be formed by changing the size of a component of the PCCP structure such that the pitch of the component becomes gradually longer toward the left and right directions away from the kick point.
The PCCP structure applied to the present invention, for instance, forms a cylindrical body in which a crease line is formed by a base common to a pair of triangles arranged in a diamond shape, where one triangle has its base contacting a base of the other triangle, and in which a fold line is formed by an oblique side in one case, or forms a cylindrical body in which a crease line is formed by a lower base common to a pair of trapezoids forming a hexagonal shape, where one trapezoid has its lower base contacting a lower base of the other trapezoid, and in which fold lines are formed by an upper base and an oblique side in another case. In such a PCCP structure, the top portions of the crease line and the fold line are formed, for instance, as obtuse angles or in arc-like shapes.
According to the structure of the golf club shaft of the present invention described above, by suitably selecting the location to which the PCCP structure is provided or the arrangement pitch of the component of the PCCP structure, a greater degree of freedom is allowed in designing the characteristics such as a kick point position in the shaft, bending stiffness distribution, bend point of the shaft, etc. without degrading the mechanical strength or increasing the material cost.
More specifically, in comparison with the conventional shaft without the PCCP structure, the shaft provided with the PCCP structure can be made thinner and lighter in weight while achieving the same strength. In addition, when formed in the same thickness, the shaft provided with the PCCP structure achieves a greater strength.
From the viewpoint of the material, the shaft provided with the PCCP structure can utilize a less expensive material of a lower strength for the same thickness when compared with a conventional shaft without the PCCP structure. Moreover, an expensive high elasticity material is not required in changing the bending stiffness (EI) distribution. Consequently, the material cost for the shaft can be reduced.
Further, the PCCP structure can be provided to form a kick point in a desired portion without changing the outer diameter or the thickness distribution. In addition, designing of a shaft with bending stiffness distribution that changes radically, which was not possible with the usual design techniques, is made possible.
Furthermore, the PCCP structure may be employed to produce shafts of original patterns and unique designs, which would appeal strongly to the consumers, arousing their willingness to purchase the products, thereby advantageously leading to the increase in the quantity of production.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.